This invention relates to a cordierite body of low thermal expansion and high porosity. The invention relates to a method of making the body which makes use of a raw material composition of coarse talc in combination with silica, an alumina yielding component, and optionally, clay. More particularly, the body is formed by extrusion. Still more particularly, the body has a honeycomb structure.
Cordierite bodies having honeycomb structures are especially suited for but not limited to use as diesel particulate filters and as substrates for catalysts and catalyst supports in converting automotive exhaust.
Honeycombs, especially extruded honeycombs, find use in a number of applications, such as diesel particulate filters, as substrates for catalytically active components for catalytic converters for automobiles, etc. Use of cordierite is favorable in these applications because of its good thermal shock resistance. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion (CTE). That is, honeycombs with low thermal expansion have good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application.
In these particular applications, in addition to low thermal expansion, a high porosity is desirable. When the body is used as a diesel particulate filter, a high volume fraction of interconnected porosity enhances the filtration capacity of the filter. When the body is used as a substrate for catalysts in a catalytic converter, a high porosity affords area for better adherence of high surface area washcoats which support the catalysts.
The following patents each relate to ceramic honeycombs having relatively low CTE's and various ranges of and types of porosity.
U.S. Pat. No. 4,869,944 relates to a cordierite honeycomb structural body to be used as a catalyst carrier having a low expansion (.ltoreq.3.times.10.sup.-7 .degree.C..sup.-1 from 40.degree. to 800.degree. C.) with limited porosity in the range of .gtoreq.30% and &lt;42%.
European Patent Publication No. 0 354 721 relates to a porous ceramic honeycomb filter used as a diesel particulate filter. Cordierite is given as the the main component. The porosity is &gt;42% . The thermal expansions are reported to be .gtoreq.7.times.10.sup.-7 .degree.C..sup.-1 from 40.degree. to 800.degree. C.
U.S. Pat. No. 3,885,977 relates to an extruded honeycomb monolithic fired ceramic whose primary crystalline phase is cordierite and whose microstructure is characterized by a greater than random orientation of the anisotropic cordierite crystallites with their low thermal expansion direction aligned substantially parallel to the plane of the webs of the monolith. The axial expansion coefficient of the honeycomb with the composition near that of stoichiometric cordierite may be as low as 5.5.times.10.sup.-7 .degree.C..sup.-1 or lower in the range of 25.degree.-1000.degree. C. Porosity values are not disclosed.
It would be desirable and an advancement in the art to be able to produce a cordierite body which has a lower thermal expansion than has been set forth by the prior patents and at the same time has a high porosity.